There has been a continuing effort to develop radar systems which are suitable for high-resolution applications, such as ground-mapping and air reconnaissance. Initially, this finer resolution was achieved by the application of pulse-compression techniques to conventional radar systems which were designed to achieve range resolution by the radiation of a short pulse, and angular, or azimuthal, resolution by the radiation of a narrow beam. The pulse-compression techniques provided significant improvement in the range resolution of the conventional radar systems, but fine angular resolution by the radiation of a narrow beam still required a large-diameter telescope which was impractical to transport with any significant degree of mobility. Subsequent to the development of pulse-compression techniques, synthetic aperture radar (SAR) techniques were developed for improving the angular resolution of a radar system to be significantly finer than that directly achievable with a radiated beam width from a conventional telescope of comparable diameter.
In prior techniques, an equivalent to a large-diameter telescope was established which was comprised of a physically long array of telescopes, each having a relatively small diameter. In the case of a long telescope array, a number of radiating elements were positioned at sampling points along a straight line and transmission signals were simultaneously fed to each element of the array. The elements were interconnected such that simultaneously received signals were vectorially added to exploit the interference between the signals received by the various elements to provide an effective radiation pattern which was equivalent to the radiation pattern of a single element multiplied by an array factor. That is, the product of a single element radiation pattern and the array factor resulted in an effective telescope pattern having significantly sharper telescope pattern lobes than the telescope pattern of the single element.
SAR systems are based upon the synthesis of an effectively long telescope array by signal processing means rather than by the use of a physically long telescope array. With an SAR, it is possible to generate a synthetic telescope many times longer than any physically large telescope that could be conveniently transported. As a result, for a telescope of given physical dimensions, the SAR will have an effective telescope beam width that is many times narrower than the beam width which is attainable with a conventional radar. In most SAR applications, a single radiating element is translated along a trajectory, to take up sequential sampling positions. At each of these sampling points, a signal is transmitted and the amplitude and the phase of the radar signals received in response to that transmission are stored. After the radiating element has traversed a distance substantially equivalent to the length of the synthetic array, the signals in storage are somewhat similar to the signals that would have been received by the elements of an actual linear array telescope.
An SAR can obtain a resolution similar to a conventional linear array of equivalent length as a consequence of the coherent transmission from the sampling points of the SAR. The stored SAR signals are subjected to an operation which corresponds to that used in forming the effective telescope pattern of a physical linear array. That is, the signals are added vectorially, so that the resulting output of the SAR is substantially the same as could be achieved with the use of a physically long, linear telescope array. Briefly, and referring now to FIG. 1, an SAR system carried by an aircraft 10 maps a target region 12 by transmitting and receiving radar signals at various sampling points S1, . . . , SN, along the flight path 14 of the aircraft. In this regard, the SAR system may be positioned in the nose portion 15 of the aircraft.
Many conventional SAR systems operate at wavelengths in the microwave band of the electromagnetic spectrum. More particularly, many conventional SAR systems operate at wavelengths between 0.8 and 100 cm. Generally, the highest resolution SAR systems operate at the shortest wavelengths, while those SAR systems operating at longer wavelengths have a somewhat lower resolution, but are less affected by errors introduced into the system, such as from vibrations in the aircraft, any element within the target field, and/or random density variation of the intervening atmosphere.
Another drawback of conventional SAR systems is that target objects in the return images of such systems, which are generated at microwave frequencies, appear much different than they do when illuminated with visible light. In this regard, such target objects may, in various instances, only be recognized by a specially trained analyst. As such, to further improve the recognizability and/or the resolution of the return images of SAR systems, and/or the radiometric efficiency of SAR systems, SAR systems are under development that operate at wavelengths in the optical band of the electromagnetic spectrum. That is, SAR systems are under development that operate at wavelengths between 0.4 micron and 10 micron. Whereas such systems may have improved resolution and/or radiometric efficiency, such systems have drawbacks. In this regard, such SAR systems are prone to errors due to the extreme sensitivity of such SAR systems to errors introduced into the system, such as from those sources described above. For example, movement of a reflective element in the target field by as little as a micron can obscure everything in a range bin of the target field (shown in FIG. 1 as R1, . . . , RM).